1. Field of the Invention
The present invention relates generally to bipolar devices having graded, reticulated, porous or interpenetrating structures, and methods of making such structures. The present invention also relates to self-organizing devices, and more particularly to combinations of materials that can spontaneously form networks resulting in bipolar devices, and methods thereof.
2. Description of the Related Art
Rechargeable batteries enjoy an enormous and constantly growing global market due to their implementation in, for example, cellular telephone, laptop computers and other consumer electronic products. In addition, the development of electrically powered vehicles represents an immense potential market for these batteries.
The lithium rechargeable battery is an attractive technology due to its comparatively high energy density, low potential for environmental and safety hazard, and relatively low associated materials and processing costs. The lithium battery is charged by applying a voltage between the battery's electrodes, which causes lithium ions and electrons to be withdrawn from lithium hosts at the battery's cathode. Lithium ions flow from the cathode to the battery's anode through an electrolyte to be reduced at the anode, the overall process requiring energy. Upon discharge, the reverse occurs; lithium ions and electrons are allowed to re-enter lithium hosts at the cathode while lithium is oxidized to lithium ions at the anode, an energetically favorable process that drives electrons through an external circuit, thereby supplying electrical power to a device to which the battery is connected.
Currently known cathode storage compounds such as LiCoO2 and LiMn2O4 when used with currently known anodes such as lithium metal or carbon have working voltages between 3 and 4V. For many applications a high voltage and low weight are desirable for the cathode as this leads to high specific energy. For example, for electrical vehicle applications the energy-to-weight ratio of the battery determines the ultimate driving distance between recharging.
Cathodes in state-of-the-art rechargeable lithium batteries contain lithium ion host materials, electronically conductive particles to electronically connect the lithium ion hosts to a current collector (i.e., a battery terminal), a binder, and a lithium-conducting liquid electrolyte. The lithium ion host particles typically are particles of lithium intercalation compounds, and the electronically conductive particles are typically made of a substance such as a high surface area carbon black or graphite. The resulting cathode includes a mixture of particles of average size typically on the order of no more than about 100 microns.
Anodes for rechargeable lithium-ion batteries typically contain a lithium ion host material such as graphite, electronically conductive particles to electronically connect the lithium ion hosts to a current collector (i.e., a battery terminal), a binder, and a lithium conducting liquid electrolyte. Alternatives to graphite or other carbons as the lithium ion host have been described by Idota et al., in Science 1997, 276, 1395, and by Limthongkul et al., in “Nanocomposite Li-Ion Battery Anodes Produced by the Partial Reduction of Mixed Oxides,” Chem. Mat. 2001.
In such cathodes or anodes, for reliable operation, good contact between particles should be maintained to ensure an electronically conductive pathway between lithium host particles and the external circuit, and a lithium-ion-conductive pathway between lithium host particles and the electrolyte.
While numerous cathode and anode compounds have been identified and are under development, a widely used system remains the LiCoO2/carbon combination first developed in the early 1990's, which has a working voltage of 3.6V. Solid polymer batteries based on polyethylene oxide (PEO) electrolyte, lithium metal anodes, and V2O5 cathodes have also been developed, but to date require elevated temperatures of 60-80° C. in order to provide sufficient power density for most applications. Lithium ion batteries based on liquid electrolytes also do not enjoy the same advantage in power density that they possess in energy density. Amongst the various rechargeable battery systems, lithium ion rechargeable have the highest energy density (typically 150 Wh/kg and 350), but comparable power densities to competing battery technologies such as Ni—Cd and Ni-MH. Energy density is intrinsically determined by the storage materials; the cell voltage being determined by the difference in lithium chemical potential between cathode and anode, while the charge capacity is the lithium concentration that can be reversibly intercalated by the cathode and anode. Power density, on the other hand, is a transport-limited quantity, determined by the rate at which ions or electrons can be inserted into or removed from the electrodes. Currently, a major limitation to the widespread use of lithium ion technology in hybrid and electric vehicles is insufficient power density and the high cost of LiCoO2.
The realizable energy and power density are enormously influenced by battery design, however. An electrode in a lithium battery that is too thick can limit discharge rate because ion transport in and out of the electrode can be rate limiting. Thus, typical high power density rechargeable batteries are of laminate construction and typically use electrodes that are of a composite mixture of active material, binder, and conductive additive. The thickness of the laminate cathode in a lithium-ion battery is typically 100-200 μm. Currently the “cell stack” consisting of two metal foil current collectors, anode, separator, and cathode, is ˜250 μm thick.
Energy density then suffers because the electrolyte, separator, and current collectors occupy a higher volume and contribute to a greater mass relative to the active material of the electrodes. Moreover, due to the need to maximize the packing density of storage material (for high energy density), the electrolyte-filled pore channels of the composite electrode are made to be tortuous and limited in cross-sectional area. Models and experiments have demonstrated that the rate-limiting transport step is in most instances Li+ ion diffusion through the liquid-filled pore channels of the composite electrode.
Solid polymer electrolytes have been described Armand et al., in “Fast Ion Transport in Solid”, P. Vashishta, J. N. Mundy and G. K. Shenoy, Eds., North-Holland, Amsterdam (1979), p. 131, describe the use of poly(ethylene oxide) and other polyetheres doped with various alkali metal salts as solid polymer electrolytes for battery applications. Subsequently, a great variety of ionically conductive solid polymer electrolytes based on a variety of lithium-ion complexing polymers have been reported (see, e.g., F. M. Gray, “Solid Polymer Electrolytes: Fundamentals and Technological Applications”, VCH, New York (1991). More recently, detailed performance characteristics of an all-solid-state LixMnO2/Li polymer battery system were reported by Sakai et al., in the Journal of Electrochem. Soc. 149 (8), A967 (2002).
High aspect ratio electrodes are described by Narang et al. in U.S. Pat. No. 6,337,156. Aspect is achieved by the use of aspected particles or flakes, which generally provide structures with inadequate geometry to meaningfully improve energy or power density.